1. Field of the Invention
The present invention relates to a four-lens-group zoom lens system and a zoom lens barrel thereof.
2. Description of the Related Art
As a four-lens-group zoom lens system of the prior art, the so-called xe2x80x9cfour-lens-group non-linearxe2x80x9d zoom lens system has been known, in which ratios of traveling distances among the lens groups are varied in the focal length range defined by the short focal length extremity and the long focal length extremity. Accordingly, a four-lens-group zoom lens system of the above type is advantageous for miniaturization of the zoom lens system and for maintaining freedom on the correcting of aberrations, since each of the four lens groups moves without being influenced by the movement of other lens groups. For example, the zoom lens systems, taught in U.S. Pat. No. 5,699,198 and Japanese Unexamined Patent Publication No. Hei-8-248319, are the four-lens-group non-linear zoom lens systems, each of which includes a positive lens group, a negative lens group, a positive lens group, and a positive lens group, in this order from the object. Furthermore, upon zooming, each lens group moves independently from other lens groups (i.e., moving the four lens groups non-linearly), thereby aberrations are suitably corrected over the entire focal length range, and miniaturization of the lens system can be attained. On the other hand, in order to move each lens group non-linearly, a cam mechanism has been required. Even when one lens group out of the four lens groups is moved linearly, a cam mechanism is still required for other three lens groups.
However, a cam mechanism is structured to move cam followers in cam grooves, and the cam followers come into contact with the cam grooves in a point-contact manner. Therefore the cam mechanism tends to be deformed due to shock occurring when the cam mechanism is being operated. If a cam groove and/or a cam follower are deformed, a smooth movement of the cam mechanism cannot be maintained, and the position of the lens group is shifted according to the amount of deformation; accordingly, an in-focus state cannot be obtained. In a conventional four-lens-group non-linear zoom lens system, since at least three cam mechanisms are required, the mechanism for zooming has inevitably become complicated, thereby it is difficult to attain high precision in the operations of the zoom lens system.
It is an object of the present invention to provide a four-lens-group zoom lens system in which the zoom mechanism thereof can be simplified, and has increased precision in operations.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in this order from the object. Upon zooming from the short focal length extremity towards the long focal length extremity, all the lens groups move towards the object so that the distance between the first lens group and the second lens group increases, the distance between the second lens group and the third lens group decreases, and the distance between the third lens group and the fourth lens group decreases. In the zoom lens system, a lens group other than the first one is arranged to be a lens group for focusing (hereinafter, a focusing lens group), and a ratio K of traveling distance of the first lens group to that of the second lens group is made constant in any part of the focal length range defined by the short focal length extremity and the long focal length extremity. Furthermore, the zoom lens system satisfies the following condition:
0.16 less than K less than 0.50xe2x80x83xe2x80x83(1) 
wherein 
K=xcex94X2/xcex94X1; 
xcex94X1 designates the traveling distance of the first lens group, upon zooming, from the short focal length extremity to the long focal length extremity; and
xcex94X2 designates the traveling distance of the second lens group, upon zooming, from the short focal length extremity to the long focal length extremity.
As explained, in the case where the ratio of the traveling distance of the first lens group to that of the second lens group is made constant, the zooming mechanism can be simplified, and precision in the operations of the zoom lens system can be increased. For example, the first lens group and the second lens group can be driven by a screw mechanism. Still further, the second lens group is arranged to be a focusing lens group.
The zoom lens system according to the present invention preferably satisfies the following condition:
1.10 less than xcex94X1/fs less than 1.45xe2x80x83xe2x80x83(2) 
wherein
fs designates the focal length of the entire zoom lens system at the short focal length extremity.
Further, the zoom lens system according to the present invention can satisfy the following condition:
0.25 less than fs/f1 less than 0.38xe2x80x83xe2x80x83(3) 
wherein
f1 designates the focal length of the first lens group.
The fourth lens group includes a biconvex positive lens element and a biconcave negative lens element, in this order from the object, and preferably satisfies the following condition:
xe2x88x920.66 less than f4p/f4n less than xe2x88x920.35xe2x80x83xe2x80x83(4) 
wherein
f4p designates the focal length of the biconvex positive lens element; and
f4n designates the focal length of the biconcave negative lens element.
In the above arrangement, the biconcave negative lens element can satisfy the following condition:
xe2x88x921 less than SF4n less than 0xe2x80x83xe2x80x83(5) 
wherein 
SF4n=(r2+r1)/(r2xe2x88x92r1); 
SF4n designates the shape factor of the biconcave negative lens element in the fourth lens group;
r1 designates the radius of curvature of the object-side surface of the biconcave negative lens element in the fourth lens group;
r2 designates the radius of curvature of the image-side surface of the biconcave negative lens element in the fourth lens group.
The zoom lens system according to the present invention can satisfy the following condition:
0.82 less than fs/f3 less than 1.10xe2x80x83xe2x80x83(6) 
wherein
f3 designates the focal length of the third lens group.
The zoom lens system according to the present invention preferably satisfies the following condition:
0.45 less than fs/f4 less than 0.56xe2x80x83xe2x80x83(7) 
wherein
f4 designates the focal length of the fourth lens group.
The present invention can also be applied to a zoom lens barrel which contains a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in this order from the object. The zoom lens barrel is arranged to move all the lens groups towards the object upon zooming from the short focal length extremity towards the long focal length extremity so that the distance between the first and the second lens groups increases, the distance between the second and the third lens groups decreases, and the distance between the third and the fourth lens groups decreases. In the zoom lens barrel, a first frame by which the first lens group is supported, and a second frame by which the second lens group is supported are moved, upon zooming, forwardly or backwardly through a linear-moving mechanism, such as screw mechanism and the like.
The linear-moving mechanism is arranged to move the first and second frames along the optical axis, with respect to a fixed frame. More concretely, in the case where a screw mechanism is employed, the screw mechanism can includes a single driving ring, helicoids respectively formed on outer and inner surfaces of the driving ring, and the corresponding helicoids respectively formed on the first and second frames, so that through the rotation of the driving ring, the first and second frames move forwardly or backwardly along the optical axis.
Instead of the above screw mechanism in which the outer-surface helicoid of the driving ring directly engages with the first-frame helicoid, and the inner-surface helicoid of the driving ring directly engage with the second-frame helicoid, another arrangement of the screw mechanism can be employed, e.g., (i) the inner-surface helicoid of the driving ring engages with a fixed frame; (ii) the driving ring and the second frame are arranged to rotate relatively, and to move together along the optical axis, and (iii) the outer-surface helicoid of the driving ring engages with the first frame. With this arrangement, if the outer-surface and inner-surface helicoids of the driving ring are formed in the same direction, the traveling distance of the first frame can be made longer.
There are various ways to non-linearly move the third lens group and the fourth lens group, with respect to the first and second lens groups which can be moved by the above-explained linear-moving mechanism. For example, the third and fourth lens groups can be arranged to non-linearly move along cam grooves which are formed on a cylindrical body which rotates in association with the driving ring or formed on the driving ring itself, while the third and fourth lens groups are linearly guided along the optical axis.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-018785 (filed on Jan. 27, 2000) which is expressly incorporated herein by reference in its entirety.